The complex trajectories and multi-target oil wells require precision placement of well borehole path and the flexibility to continually maintain path control. It is preferred to control or “steer” the direction or path of the borehole during the drilling operation. It is further preferred to determine and to control the path rapidly during the drilling operation at any depth and target as the borehole is advanced by the drilling operation.
Directional drilling is complicated by the necessity to operate a drill bit steering device within harsh borehole conditions. The steering device is typically disposed near the drill bit, which terminates a lower or “down hole” end of a drill string. In order to obtain the desired real time directional control, it is preferred to operate the steering device remotely from the surface of the earth based upon real time measures of borehole azimuthal offset and curvature. Furthermore, the steering device must be operated to maintain the desired path and direction while being deployed at possibly a great depth within the borehole and while maintaining practical drilling speeds. Finally, the steering device must reliably operate under exceptional heat, pressure, and vibration conditions that can be encountered during the drilling operation.
Many types of directional steering devices, comprising a motor disposed in a housing with an axis displaced from the axis of the drill string, are known in the prior art. The motor can be a variety of types including electric, or hydraulic. Hydraulic turbine motors operated by circulating drilling fluid are commonly known as “mud” motors. A rotary bit is attached to a shaft of the motor, and is rotated by the action of the motor. The axially offset motor housing, commonly referred to as a bent subsection or “bent sub”, provides axial displacement that can be used to change the trajectory of the borehole. By rotating the drill bit with the motor and simultaneously rotating the drill bit with the drill string, the trajectory or path of the advancing borehole is parallel to the axis of the drill string. By rotating the drill bit with the motor only, the trajectory of the borehole is deviated from the axis of the drill string. By alternating these two methodologies of drill bit rotation, the path of the borehole can be controlled. A more detailed description of directional drilling using the bent sub concept is presented in U.S. Pat. Nos. 3,713,500, 3,841,420 and 4,492,276, which are herein entered into this disclosure by reference.
The prior art contains methods and apparatus for adjusting the angle of “bend” of a bent sub housing thereby directing the angle of borehole deviation as a function of this angle. The prior art also contains apparatus and methods for dealing with unwanted torques that result from steering operations including clutches that control relative bit rotation in order to position the bit azimuthally as needed within the walls of the borehole. Prior art steering systems using variations of the bent sub concept typically rely upon complex pushing or pointing forces and the associated equipment which directs the hole path by exerting large pressures on the bit perpendicular to the borehole path while rotating the drill string. These forces are often obtained using hydraulic systems that are typically expensive and present additional operational risks in the previously mentioned harsh drilling environment. Furthermore, these perpendicular forces typically require the steering device to be fabricated with mechanically strong components thereby further increasing the initial and operating cost of the steering device.
U.S. patent application Ser. No. 11/848,328, which is entered into this disclosure by reference, discloses apparatus and methods for steering the direction of a borehole advanced by cutting action of a rotary drill bit terminating a lower or “down hole” end of a drill string. The rotation speed of the bit is periodically varied during a rotation of the drill string thereby cutting a disproportionately larger amount of material from an azimuthal arc of wall of the borehole, which will results in an azimuthal deviation in borehole direction. The steering device, which is disposed at the downhole end of a drill string, comprises a motor disposed in a bent housing subsection or “bent sub”. A rotary drill bit is attached to a shaft of the motor. The drill bit can be rotated by both the motor and by the rotary action of the drill string. To deviate the borehole, rotation rate of the bit is periodically slowed or “pulsed” in a predetermined arc thereby cutting a disproportionally small amount of material from the borehole wall. As a result, the bit moves to the opposite side of the borehole and cuts disproportionately larger amount of material from the borehole wall. The borehole then tends to deviate and advance in the azimuthal direction in which the disproportional large amount of borehole wall material has been removed. This methodology is referred to as a Pulsed Modulated Steering.
Regardless of the steering system employed, the effective direction of a borehole to a predetermined target requires reliable, accurate, precise, and preferably real time measures of the azimuthal offset and curvature of the borehole at or very near the drill bit.